This is an often requested feature, and is at the top of my list for improvements to the game features.
Do a search for "order of battle menu" and you will find various threads on it.
Essentially an army list similar to the province list where you can select various armies and see their status, view them, and give them orders. Also being able to select multiple armies easily and give an order that will merge them together would be handy.
In the case of planes being able to select multiple groups of planes that are patrolling in one area and give them a different patrol order is a much needed improvement for the mobile app. I find precise selection of units to be time consuming and frustrating on mobile myself.
The ability to assign several groups to a "hotkey" such as "army A" with the menu would be additionally helpful in certain situations, particularly planes, where you want several units to do a certain task simultaneously. If you could give "army A" an order and then split the units up into individual groups, but retain the ability to select them all with the "army A" button later that will also be useful. Mop up a country and then give all of your units an order to go somewhere else with one simple selection.
